My computer sits in a utility closet near the cable TV junction box. One of the unused cables terminates near my Denon receiver. My soundcard has an SPDIF coax output (looks like an RCA jack). Can I route that signal over the cable TV cable to my reciever?

I would guess that the cable length to the receiver is 25 feet at most.

The connectors don't match of course. Is there some kind of adapter I could use?

Thanks!

Yes you can use cable tv coax (often RJ6). Just hit up Radio Shack for a couple of F-type to RCA adapters.